This invention relates to the production of printing ink.
A printing ink is conventionally made by charging the dry or flushed pigment into an ink mixing tank, adding additional components, mixing well and finishing the ink by either running the ink through a shot or bead mill and filtering, or over a roll mill and possibly filtering or simply filtering out of the mixing tank. In any event, it is essential that the pigment be uniformly dispersed in the vehicle.
The pigments used are usually formed at a low concentration (2-5 percent) in an aqueous suspension. These are then concentrated in a plate and frame filter press or by flocculation, flotation, or settling. The pigment in this concentrated form is generally referred to as presscake. There are many techniques for improving dispersibility of pigments such as adding the pigment in the form of a fine powder generally produced by drying in an oven or on a heated belt or in a spray dryer followed by grinding, preferably in a powder mill, and blending. Such process agglomerates the fine pigment particles formed upon synthesis. Many of the agglomerates cannot be broken down economically. Further, powdered pigments are difficult to redisperse to obtain maximum dispersion.
While conventional pigment presscake (i.e. presscake comprising greater than 50 percent water, based on weight of presscake) disperses more readily in an ink composition than dry pigment powders, nevertheless, due to the low solids content of the aqueous cake generally obtained by conventional processes, dispersion of untreated presscake in inks is not economically feasible because of the low pigment loading. Also, the amount of water carried by the conventional presscake into the oil ink presents problems. This water must be substantially removed during ink manufacturing. This removal process, in addition to considerably lengthening the manufacturing process time, is complicated by the effect of the emulsified water which disrupts the flow properties of these inks making handling and further processing difficult.
Accordingly, the pigment is often "flushed" to transfer the pigment particles from the aqueous phase to a non-polar oil or resin phase. This assists in keeping pigment particles non-agglomerated and easier to dilute for ultimate use in inks or coatings. Further, dusting is nonexistent using the flushed paste while it is usually extreme with dry pigments. Since the dry pigment has many agglomerates which cannot be broken down economically by subsequent dispersing equipment, inks from these are usually more opaque, less glossy, and more pigment is required to gain a unit amount of coloration (less color strength).
Flushing has been carried out for many decades by mixing pigment presscake with suitable types and quantities of "vehicles" which may be an oil, or a solution of resin or resins and other additives in a solvent. In the prior art, the transfer was effected by merely kneading the presscake and vehicle together until a major portion of the water has separated. Periodically the flushers have to be tilted to remove water as it breaks, i.e., becomes freed of pigment which has transferred to the oil base. However, flushing has the disadvantage of being time consuming and expensive. Accordingly, it is a purpose of the instant invention to produce a printing ink directly from a phase-transferred presscake and thus avoid the disadvantage of either dried or flushed pigments.
U.S. Pat. No. 4,230,610, issued Oct. 28, 1980, to Falcione et al, assigned to Calgon Corporation, discloses the use of polyacrylates as dispersants for aqueous magnesium oxide pigment slurries prior to dewatering. The polyacrylate is obtained by neutralizing poly(acrylic acid) to a pH of from about 8 to about 12 with bases such as sodium or ammonium hydroxide. This changes the solubilization process properties of the polyacrylic acid resin from oleophilic to hydrophilic.